Imagine you are playing a crafting game like Minecraft or a building game where you combine items to create new tools or potions. You start with some basic ingredients, mix or heat them, and suddenly you have something completely different—stronger, brighter, or more useful. That is very similar to what happens in real-life chemistry. When substances react chemically, the tiny particles inside them rearrange to form new substances with new properties.
This lesson explores how and why substances react in characteristic ways, what happens to atoms during chemical reactions, and how these processes show up all around you—in cooking, batteries, rusting bikes, fireworks, and even inside your own body.
All matter—everything that has mass and takes up space—is made of atoms. Atoms are extremely small particles that act like the “letters” of the material world. Just as you can arrange letters to make different words, you can arrange atoms to make different substances.
Key ideas about atoms and molecules:
When substances react chemically, their atoms do not disappear and new atoms do not appear out of nowhere. Instead, the atoms rearrange to form new molecules.
The idea that atoms are rearranged but not created or destroyed in a chemical reaction is part of the law of conservation of mass.
When you look at particles before and after a reaction, you can imagine them moving around and reconnecting, as shown in [Figure 1].
Before digging deeper into chemical reactions, it is important to tell them apart from physical changes.
Physical change: The substance changes form or appearance, but its molecules stay the same.
Chemical change (chemical reaction): The atoms are rearranged and new substances form with new properties.
To decide whether a change is physical or chemical, ask: Are new substances with new properties formed? If yes, it is a chemical change.
How can you tell when a chemical reaction is happening? Scientists look for evidence called indicators of chemical change. One sign alone is not always proof, but several signs together strongly suggest a chemical reaction.
Common signs include:
Many of these signs are visible at the macroscopic level (what you can see, touch, or hear). But what is really happening is at the microscopic level: atoms are rearranging to form new substances.
During a chemical reaction, the atoms in the reactants break their old bonds and form new bonds, creating new molecules called products.
Important idea: The number of atoms of each element stays the same before and after the reaction, even though the atoms are in different combinations. The atoms are like LEGO bricks—you can take them apart from one build and rearrange them into a new design, but you still have the same number of each brick type.
For example, consider the reaction between hydrogen gas and oxygen gas to form water. At the particle level, hydrogen molecules and oxygen molecules collide, bonds break, and new bonds form between hydrogen and oxygen atoms, as seen in [Figure 2].
Even though the molecules change, the total number of hydrogen atoms and oxygen atoms remains constant. This is why mass is conserved in a chemical reaction, if everything is contained.
Scientists use chemical equations to represent what happens in a reaction. A simple example is the reaction of hydrogen and oxygen to form water:
hydrogen + oxygen → water
Written with chemical formulas, one balanced version is:
2H2 + O2 → 2H2O
Here is what this means:
Count the atoms on each side:
The numbers of hydrogen and oxygen atoms stay the same; only their groupings change.
The most important idea in this topic is that new substances formed in chemical reactions have different properties from the reactants. Those new properties come from the new combinations and structures of atoms in the product molecules.
Consider these examples:
Even though the reactants and products contain some of the same elements, the arrangement of atoms in molecules changes, creating new properties. This is like rearranging the same LEGO bricks into a boat instead of a car—you get a structure with different abilities, even though you used the same pieces.
Your kitchen is a tiny chemistry lab. When you cook food, many chemical reactions change flavors, color, and texture.
Once a cake is baked, you cannot easily get back the raw eggs, sugar, and flour. That is a sign of a chemical change with new substances and properties.
If you leave a bike outside in the rain, you might notice rusty spots forming. Rusting is a slow chemical reaction between iron, oxygen in the air, and water.
Overall, iron atoms in the bike react with oxygen molecules and water to form new compounds (iron oxides). These compounds are weaker and crumbly, so the bike becomes less sturdy. The atoms are still there, but now they are rearranged into rust instead of strong metal.
Rusting shows why understanding chemical reactions matters in real life: engineers design paints and coatings to prevent or slow these reactions, so bridges, cars, and buildings last longer.
Burning is a fast, energy-releasing chemical reaction called combustion. It usually involves a fuel reacting with oxygen to produce new substances and energy.
In all these examples, the properties change dramatically. Solid wood or liquid fuel turns into gases, ashes, and heat energy.
Your body is running thousands of chemical reactions every second. These reactions are called metabolism.
In cellular respiration, the overall reaction can be summarized (in a simplified way) like this:
glucose + oxygen → carbon dioxide + water + energy
Again, atoms are rearranged from one set of molecules into another, with very different properties and functions.
Many textbooks represent energy changes in a reaction diagram that shows reactant energy levels and product energy levels, like the one in [Figure 3].
Chemical reactions always involve energy. Bonds between atoms store energy, and when these bonds break and new bonds form, energy can be released or absorbed.
Even when the temperature change is small, energy is still involved in rearranging atoms. That is part of why chemical reactions power so many technologies, from batteries in your phone to rocket engines.
Here are some simple ways to see chemical reactions in action. Always get adult permission and follow safety instructions.
Materials:
Steps:
What you observe:
This is a chemical reaction with new substances and new properties.
Red cabbage juice can act as an indicator, changing color when it reacts with acids or bases.
Materials:
Steps:
What you observe:
These reactions show that substances can react in characteristic ways (acids vs bases) and create visible changes.
Not all substances react in the same way. Some react very easily and quickly; others react slowly or almost not at all. The way a substance reacts is part of its chemical properties.
These chemical properties depend on the types of atoms in the substance and how they are bonded together in molecules or crystals.
Across all these examples, one big pattern appears:
This is what makes chemistry powerful. By understanding how atoms regroup, scientists and engineers can design new materials: lighter metals for planes, stronger plastics for sports gear, colorful dyes for clothes, life-saving medicines, and batteries that last longer in your devices ⚡.
Substances react chemically in characteristic ways because their atoms and bonds are arranged in specific patterns. In a chemical reaction, these atoms are rearranged into new molecules, but the total number of each type of atom stays the same. This rearrangement of atoms and bonds leads to the formation of new substances with new properties—different colors, states, strengths, flammability, or reactivity—compared with the original reactants. Chemical reactions show up everywhere: in cooking, rusting, burning, body processes, and technology. By recognizing the signs of chemical change (like color change, gas formation, energy change, or precipitates) and understanding that atoms are conserved while molecules change, you can explain how ordinary materials transform into something completely different, just like crafting in your favorite game but in the real, atomic-sized world.
